1. Field of the Invention
The invention relates to an apparatus and method that determines whether an abnormality has occurred in an automatic transmission mounted in a vehicle. More particularly, the invention relates to an apparatus and method that accurately determines whether a gear ratio corresponding to a specified gear speed is not established.
2. Description of the Related Art
An automatic transmission mounted in a vehicle is a combination of a torque converter and a gear type shift mechanism. This automatic transmission is structured so as to automatically establish a gear speed according to an operating state of an engine by selectively switching between applying and releasing a plurality of friction apply elements such as clutches and brakes and thus changing the power transmission path in the gear type shift mechanism. This automatic transmission is provided with a hydraulic pressure control circuit which controls the supply of hydraulic fluid to the friction apply elements so as to apply or release those friction apply elements.
This automatic transmission is structured such that the gear speed, i.e., shifting, is controlled by controlling the hydraulic pressure supplied to the friction apply elements with the hydraulic pressure control circuit. This hydraulic pressure control circuit is provided with various solenoid valves which perform various functions such as generating hydraulic pressure, supplying and releasing the hydraulic pressure, and regulating the hydraulic pressure. The operation of these solenoid valves is controlled by electric control signals to control the hydraulic pressure supplied to the friction apply elements and the like.
In an automatic transmission having this kind of structure, there is a possibility that a solenoid valve may fail. For example, there may be an electrical failure, e.g., a disconnection or short circuit, in the solenoid valve, or there may be a mechanical failure, e.g., the plunger may stick or a seal may fail due to foreign matter wearing away at the seal. These kinds of failures may very well result in the solenoid valve failing to work properly. Thus, if such a failure occurs, a specified gear speed may not be able to be established due to the desired friction apply element not applying or releasing as it should in response to a shift command output according to the operating state of the engine.
Japanese Patent Application Publication No. JP-A-11-280898 describes a control apparatus of an automatic transmission which, in the event of a failure, appropriately and logically performs a failsafe control that prohibits the gear speed in which the failure was detected from being established and changes the transmission to another gear speed. This control apparatus of an automatic transmission includes a torque converter, a gear type shift mechanism that inputs power from an internal combustion engine via the torque converter, a plurality of friction apply elements that switch the power transmission path in the gear type shift mechanism, and a hydraulic pressure control circuit that switches gear speeds of the gear type shift mechanism by controlling the supply and release of hydraulic pressure to and from the friction apply elements. The control apparatus of the automatic transmission described above also includes actual gear ratio computing means for computing an actual gear ratio based on an input rotation speed and an output rotation speed of the gear type shift mechanism, shift command outputting means for outputting a shift command according to the operating state of the internal combustion engine, gear failure detecting means for detecting a gear failure in which a gear ratio of the gear type shift mechanism does not establish as directed by the shift command, by comparing the actual gear ratio with a gear ratio of a target gear speed specified by a shift command output by the shift command outputting means, and vehicle speed detecting means for detecting a vehicle speed. The gear type shift mechanism is structured to fall into a neutral state if a gear failure occurs in a predetermined high gear speed. Also, when a gear failure is detected in a predetermined high gear speed by the gear failure detecting means and the vehicle speed at which the gear failure was detected by the vehicle speed detecting means is less than a predetermined vehicle speed, the shift command outputting means described above changes the shift command to one for a predetermined gear speed that is lower than the high gear speed and which has a gear ratio that is closest to the gear ratio of the predetermined high gear speed. On the other hand, when a gear failure is detected in a predetermined high gear speed by the gear failure detecting means and the vehicle speed at which the gear failure was detected by the vehicle speed detecting means is greater than the predetermined vehicle speed, the shift command outputting means maintains the current shift command. When the vehicle speeds drops lower than the predetermined vehicle speed, shift command outputting means then changes that shift command to one for a predetermined low gear speed.
This control apparatus of an automatic transmission can shift the automatic transmission into a gear speed that can be established when it is determined that a failure has occurred in which the automatic transmission falls into a neutral state despite a shift command being output. As a result, a situation in which the automatic transmission is kept in the neutral state can be avoided so that driving force can be transmitted to the driven wheels by shifting the automatic transmission into a gear speed that can be established.
In the technology described in Japanese Patent Application Publication No. JP-A-11-280898, the actual gear ratio is calculated from the input shaft rotation speed and the output shaft rotation speed of the automatic transmission. An abnormality in which there is a gear failure where a gear ratio according to a shift command is unable to be established (i.e., a neutral failure or a case in which a gear speed is established that is different from the specified gear speed) can be detected when the actual gear ratio is outside of the gear ratio range of the specified gear speed (this range is set beforehand based on the detection accuracy of a rotation speed sensor and the like). When the output shaft rotation speed is in the high rotation speed region, the region in which this type of abnormality is unable to be detected tends to become larger, which leads to reduced accuracy of abnormality detection. Japanese Patent Application Publication No. JP-A-11-280898, however, makes no mention of this kind of problem.